Caustic SCC of Inconel 625? 1

[kevlar49]

We have an Inconel 625 LCF bellows on the feed side of a feed/effluent exchanger (feed:100F /effluent: 700F). We recently had a failure that brought our plant down. Visual inspection of the bellows indicated highly branched cracking suggesting SCC. We know that we occasionally have caustic carryover events and that we have failed several Inconel 625 LCF bellows in the past.

Now I know that Inconel 600 can crack in highly caustic environments, but does anyone know for certain that Inconel 625 can crack in caustic environments?

 

[EdStainless]

Well, I know that you can caustic stress crack 600, which is 72% Ni. 625 is supposed to be resistant to 50% caustic, but higher concentrations could be trouble.

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[strider6]

Kevlar,

What is the main composition of the fluid?

S.

http://www.corrosionist.com

 

[kevlar49]

strider6,

Under normal conditions: Mostly natural gas and hydrogen (0.8 mole % H2, C1-C5 hydrocarbons, possibly up to 3 vol% CO or CO2, and trace (1/2 grain in 100 cubic feet).

This feed is routed through a caustic wash water column to remove H2S and chlorides prior to entering the reaction system and before coming into contact with this exchanger. Unfortunately, occassionally there are upsets which allow highly concentrated caustic to enter the tube side of this exchanger. The bellows is on the floating head.

Concern is with highly concentrated caustic leaving deposits behind on the bellows. Shell side is 700 F so, water will easily evaporate when touching this surface.

 

[kevlar49]

Sorry, just noticed that I forgot to mention that this was in a hydrogen manufacturing plant (Steam Methane Reformer).

 

[strider6]

kevlar

625 alloy can be susceptible to caustic SCC. You can find useful article on caustic corrosion and a table with limits for various alloy (also 625 alloy)in the ref below.
It seems that there are few data for 625 alloy in NaOH service, and that the a safe limits is "50% caustic at atmospheric boiling (300F)" and that for higher conc and T is should be tested before use.
On the NACE Corrosion Survey Database there are data up to 75% conc and 325°F with a corrosion rate of <20 mpy.
So considering these limits and the aspect of the failure is possible to suppose that your failure is due to a Caustic SCC.


ref:

http://www.hghouston.com/naoh_tbl.html

http://www.hghouston.com/naoh.html

hope this help

S.

http://www.corrosionist.com

 

[kevlar49]

thanks strider6,

By the way it also mentions C-276 is in the same category as 625. Prior to receiving this info, I recommended C-276 because I hadn't thought it was scc prone. Any experience with caustic cracking of c276?